Because I’m too lazy for deep ones (I spent all day watching stuff for the time-travel book).

Nonfiction can be divided into stories and nerd books. A story book is nonfiction that actually has a story arc: The Lost City of Z, The Immortal Life of Henrietta Lacks, The Plutonium Files, The Burglary. It’s not just a story—a good book will have lots of backstory and context for events—but there is a timeline, an arc, and people who make it interesting. City of Z covers an Amazonian expedition by Percy Fawcett, and the seekers who followed him. Plutonium Files deals with American nuclear and medical research on unwitting patients. The Burglary covers a break-in at an FBI office and the firestorm of revelations that resulted. Most biographies fit in here.

A nerdbook is one that doesn’t have a narrative spine. The English Town, which I’m reading at the moment, is a great book about the forces that shape the way towns develop and their architecture, but while there are multiple stories within it, it doesn’t have an overall arc. The Creationists and Bloodless Revolution both have a story arc as they follow different ideas (creationism and vegetarianism respectively) but don’t have a core protagonist or the kind of conflict that makes a story book arresting. My movie reference books fall into this category.

Which type of book you’re writing usually depends on what subject material you’re working with. It is possible to take a gripping story and turn it into a dull recitation of fact. It is not possible to take material that’s not riven with conflict and honestly infuse some into it.

And of course having a gripping story won’t help if you’re not interested in the material. I’m unlikely to respond to a sports biography, even if the protagonist did have an interesting life.